Switching converters are widely used in the power supply field because of their high conversion efficiency. Yet, a switching converter has a phase margin that generally should be compensated, and the output capacitor and associated equivalent series resistor (ESR) of a converter generally create a zero in the transfer function of the converter's system that can lead to instability. To correct these problems, controllers may compensate a loop system with additional components that boost the phase margin and prevent the instability that could be created.
Generally, a controller for a switching converter is designed and manufactured in a single package. Such a controller may include an op-amp and a pulse-width modulator (PWM). As with any package, the number of pins on the package impacts the cost of the device. A lower number of pins generally will reduce the cost of the device.
Ideally, the controller would have fixed compensation for the switching converter so no additional circuitry would be needed off the package, and the need for pins to accommodate the compensation circuit would be eliminated. The disadvantage to this approach is inflexibility in allowing for various values of the output capacitor and ESR. Generally, if the output capacitor and ESR create a zero with a frequency less than the unity gain frequency, the system will become instable.
Also, the controller package could allow for compensation circuitry off the package so that a circuit designer would be able to consider a known value of the output capacitor and ESR when creating the compensation network. One disadvantage of conventional methods using this approach is that at least two pins on a package are generally required for the compensation network. The increased pins generally increase cost of the package.